Unit magnification optical system with improved reflective reticle

Abstract

An optical projection system has been provided which is particularly suited for use in microlithography and includes a source of exposure energy for generating a beam of energy. A primary lens and mirror are located in the path of the beam for receiving the beam and passing only a portion of the beam therethrough. A refractive lens group is located in the path of the portion of the beam for receiving and transmitting that portion. A recticle element is located in the path of the portion of the beam and has a uniform thickness having a pattern on one surface thereof and an unpatterned portion adjacent thereto. The reticle element is positioned for permitting the portion of the beam to pass through its thickness and for reflecting the portion of the beam back through its thickness and the refractive lens group to the primary lens and mirror. The primary lens and mirror is positioned to receive the reflected beam and includes a surface for reflecting a portion of the reflected beam back through the refractive lens group and the unpatterned portion on the reticle element to a surface to be imaged. The reticle element includes a transparent element having a predetermined uniform thickness and having two parallel planar surfaces separated by that thickness. The reticle pattern on one of the planar surfaces is made from a material having a high degree of reflectivity with respect to optical exposure energy. The reticle further includes a covering physically contacting the reticle pattern and completely covering that pattern. The covering has an index of refraction with respect to optical exposure energy which is sufficiently lower than the reticle index of refraction to cause a small portion of the exposure energy striking the covering to be reflected with insufficient intensity to expose photoresist on a wafer but with sufficient intensity and phase shift to improve edge control and resolution of the projected image formed by the reflected reticle pattern.

What is claimed is: 
     
       1. A reticle for use in an optical projection system comprising: a transparent element having a first index of refraction and having a predetermined uniform thickness and surfaces separated by said thickness;   a reticle pattern on one of said surfaces made from a material having a high degree of reflectivity with respect to optical exposure energy; and   means physically contacting said reticle pattern for completely covering said reticle pattern, said covering means having a high degree of absorption with respect to optical exposure energy and a second index of refraction, said second index of refraction being lower than said first index of refraction.   
     
     
       2. A reticle as defined in claim 1 wherein said second index of refraction is in the range of 7 to 14% lower than said first index of refraction. 
     
     
       3. A reticle as defined in claim 2 wherein said first index of refraction is approximately 1.5 and said second index of refraction is in the range of 1.29 to 1.40. 
     
     
       4. A reticle as defined in claim 1, wherein said reticle pattern is aluminum, said transparent element is fused silica having a thickness of less than 0.5 inches, and said covering means is a silicone polymer. 
     
     
       5. A reticle as defined in claim 1 including a plurality of patterns each having a predetermined external dimension and a corresponding plurality of transparent spaces adjacent each pattern with the dimensions of each space being equal to or greater than the dimensions of its adjacent pattern in all respects. 
     
     
       6. A photolithographic projection optical system comprising: a source of exposure energy for generating a beam of energy;   an optical element located in the path of said beam for receiving said beam of energy and passing only a portion of said beam therethrough;   a refractive lens group located in the path of said portion of said beam for receiving and transmitting said portion of said beam of energy;   a reticle element located in the path of said portion of said beam, said reticle element including a transparent portion having a uniform thickness and having a pattern on one surface thereof, and an unpatterned portion adjacent thereto, said pattern being made from a material having a high degree of reflectivity with respect to optical exposure energy for reflecting said portion of said beam striking said pattern at a first intensity level and means completely covering said pattern for reflecting and changing the phase of said portion of said beam striking said covering means at a second intensity level significantly lower than said first intensity level, said reticle element being positioned for permitting said portion of said beam to pass through said thickness and for reflecting said portion of said beam through said thickness, and said lens group to said optical element;   said optical element receiving said reflected beams and including means for reflecting a portion of said reflected beams back through said lens group and said unpatterned portion of said reticle element to a surface to be imaged.   
     
     
       7. An optical system as defined in claim 6 wherein said transparent portion of said reticle element has a first index of refraction and wherein said covering means has a second index of refraction, said second index of refraction being lower than said first index of refraction. 
     
     
       8. An optical system as defined in claim 7 wherein said second index of refraction is in the range of 7 to 14% lower than said first index of refraction. 
     
     
       9. An optical system as defined in claim 8 wherein said first index of refraction is approximately 1.5 and said second index of refraction is in the range of 1.29 to 1.40. 
     
     
       10. An optical system as defined in claim 6 wherein said reticle pattern is aluminum, said transparent element is fused silica having a thickness of less than 0.5 inches, and said covering means is a silicone polymer. 
     
     
       11. A optical system as defined in claim 6 including a plurality of patterns each having a predetermined external dimension and a corresponding plurality of transparent spaces adjacent each pattern with the dimensions of each space being equal to or greater than the dimensions of its adjacent pattern in all respects. 
     
     
       12. A photolithographic projection optical system including: a source of exposure energy for generating a beam of energy;   a lens assembly for receiving and transmitting energy beams from said energy source, said lens assembly having a reticle on an end thereof farthest from said energy source and located in the path of said beam of energy, said reticle having a pattern thereon on a surface farthest from said energy source and an unpatterned portion adjacent thereto, said reticle being positioned for receiving said energy beam through said lens assembly and reflecting said pattern back through said reticle and said lens assembly, said reticle element including a transparent portion having a uniform thickness and having a pattern on one surface thereof, and an unpatterned portion adjacent thereto, said pattern being made from a material having a high degree of reflectivity with respect to optical exposure energy for reflecting said beam striking said pattern at a first intensity level and means completely covering said pattern for reflecting and changing the phase of said beam striking said covering means at a second intensity level significantly lower than said level, said recticle element being positioned for permitting said beam to pass through said thickness and for reflecting said beams at said first and second intensity levels through said thickness, and said lens assembly;   a mirror for receiving said reflected beams from said lens assembly and reflecting said beams back through said lens assembly and said unpatterned portion of said reticle onto a surface to be imaged.